Various oxazole compounds have been developed and are disclosed in documents such as WO 03/072102, WO 98/15274, etc. However, the oxazole compounds of the present invention are not disclosed in any literature.
Some compounds having a specific inhibitory action against phosphodiesterase 4 (PDE4) have been reported. However, known PDE4 inhibitors have problems of side effects such as vomit induction, nausea, etc. and/or a defect of insufficient PDE4 inhibitory action. Therefore, known PDE4 inhibitors are not clinically used as therapeutic agents.